Image forming apparatus

ABSTRACT

An image forming apparatus includes a stacking unit, a pickup roller, a conveying roller, a transfer unit, a motor, a determiner, and a velocity adjuster. The conveying roller conveys, in a conveying direction, a sheet fed by the pickup roller from the stacking unit. The determiner determines a value of a parameter corresponding to a load torque applied to a motor rotor. Based on a length between a front end of the sheet and a nip position of the conveying roller at a first timing when the determined value changes from a value smaller than a predetermined value to a value greater than the predetermined value, and a length between the nip position and a predetermined position downstream of the conveying roller and upstream of an image forming position in the conveying direction, the velocity adjuster adjusts a conveying velocity at which the sheet is conveyed to the predetermined position.

BACKGROUND Field

The present disclosure relates to an image forming apparatus thatadjusts a conveying velocity of a sheet being conveyed.

Description of the Related Art

Conventionally, there has been an image forming apparatus configured todetect, based on a change in a load torque (a load fluctuation) appliedto a rotor of a motor for driving conveying rollers conveying a sheet,whether the front end of the sheet reaches (passes through) a nipportion of the conveying rollers (Japanese Patent Application Laid-OpenNo. 2000-238934).

Further, there also has been an image forming apparatus configured toadjust, based on the detection result of a sensor provided in aconveying path, the conveying velocity of a sheet so that the sheet isconveyed according to an image forming sequence set in advance (so thatthe sheet is conveyed to an image forming position at an appropriatetiming). Specifically, the image forming apparatus adjusts based on thedetection result of a sensor, the conveying velocity of a sheet so thatthe sheet reaches a target position at a timing determined in advance.

In the configuration of the image forming apparatus discussed inJapanese Patent Application Laid-Open No. 2000-238934, the timing fordetecting the load fluctuation is determined as the timing when thefront end of the sheet reaches the nip position of the conveyingrollers. Actually, at the timing when the load fluctuation is detected,however, the front end of the sheet is located upstream of the nipposition of the conveying rollers due to the thickness of the sheet.

If the conveying velocity is adjusted in the above manner in the statewhere the timing for detecting the load fluctuation is determined as thetiming when the front end of the sheet reaches the nip position of theconveying rollers, the following issue may occur. Specifically, even ifthe conveying velocity is adjusted so that the front end of the sheetreaches a target position at a timing determined in advance, theposition of the front end of the sheet at this timing may be locatedupstream of the target position. As a result, the sheet may reach animage forming position after the timing when the formation of an imageon the sheet is started. As a result, the image may not be formed at anappropriate position on the sheet.

SUMMARY

The present disclosure is directed to preventing the situation where animage is formed at an inappropriate position on a sheet.

According to an aspect of the present disclosure, an image formingapparatus includes a stacking unit on which a sheet is to be stacked, apickup roller configured to feed the sheet stacked on the stacking unit,a first conveying roller configured to convey the sheet fed by thepickup roller, a transfer unit configured to transfer an image onto thesheet at an image forming position downstream of the first conveyingroller in a conveying direction in which the sheet is conveyed, a motorconfigured to drive the first conveying roller, a determiner configuredto determine a value of a parameter corresponding to a load torqueapplied to a rotor of the motor, and a velocity adjuster configured to,based on a length between a position of a front end of the sheet and anip position of the first conveying roller at a first timing when thevalue of the parameter determined by the determiner changes from a valuesmaller than a predetermined value to a value greater than thepredetermined value, and a length between the nip position of the firstconveying roller and a predetermined position downstream of the firstconveying roller and upstream of the image forming position in theconveying direction, adjust a conveying velocity at which the sheetbeing conveyed at a predetermined velocity by the first conveying rolleris conveyed to the predetermined position.

Further features of the present disclosure will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating an image formingapparatus.

FIG. 2 is a block diagram illustrating a control configuration of theimage forming apparatus.

FIG. 3 is a diagram illustrating a relationship between a two-phasemotor including an A-phase and a B-phase, and a rotating coordinatesystem represented by a d-axis and a q-axis.

FIG. 4 is a block diagram illustrating a configuration of a motorcontrol device.

FIG. 5 is a diagram illustrating a configuration for detecting a fedrecording medium.

FIG. 6 is a diagram illustrating a deviation Δθ in a case where thinpaper is conveyed and a deviation Δθ in a case where thick paper isconveyed, according to a first exemplary embodiment.

FIGS. 7A and 7B are diagrams illustrating a position of a front end of arecording medium at a timing when a sheet detector outputs a signal ‘1’(a timing when the recording medium is detected), according to the firstexemplary embodiment.

FIG. 8 is a diagram illustrating a relationship between a grammage of arecording medium to be conveyed, and a distance from the position of thefront end of the recording medium at the timing when the sheet detectoroutputs the signal ‘1’ to conveying rollers.

FIG. 9 is a flowchart illustrating a control method for controlling aconveying velocity V by a central processing unit (CPU).

FIG. 10 is a diagram illustrating a relationship between a grammage of arecording medium to be conveyed, and time Tc from when the recordingmedium is detected to when a front end of the recording medium reaches anip position n.

FIGS. 11A and 11B are diagrams illustrating a position of a front end ofa recording medium at a timing when a sheet detector outputs a signal‘1’, according to a third exemplary embodiment.

FIG. 12 is a diagram illustrating a state of a deviation Δθ according tothe third exemplary embodiment.

FIG. 13 is a diagram illustrating a relationship between a grammage ofthe recording medium to be conveyed, and a distance Lc from the positionof the front end of the recording medium at the timing when the sheetdetector outputs the signal ‘1’ to conveying rollers.

FIG. 14 is a block diagram illustrating a configuration of a motorcontrol device that performs velocity feedback control.

DESCRIPTION OF THE EMBODIMENTS

Exemplary embodiments of the present disclosure will be described belowwith reference to the drawings. The shapes and the relative arrangementof components described in these exemplary embodiments, however, shouldbe appropriately changed depending on the configuration of an apparatusto which the present disclosure is applied and various conditions, andthe scope of the present disclosure is not limited to the followingexemplary embodiments. In the following description, a case is describedwhere a motor control device is provided in an image forming apparatus.The motor control device, however, is provided not only in an imageforming apparatus. For example, the motor control device is also used ina sheet conveying apparatus that conveys a sheet such as a recordingmedium or a document.

[Image Forming Apparatus]

A first exemplary embodiment will be described below. FIG. 1 is across-sectional view illustrating the configuration of a monochromeelectrophotographic copying machine (hereinafter referred to as “imageforming apparatus”) 100 that includes a sheet conveying apparatus usedin the present exemplary embodiment. The image forming apparatus 100 isnot limited to a copying machine, and may be, for example, a facsimileapparatus, a printing machine, or a printer. A recording method is notlimited to an electrophotographic method, and may be, for example, aninkjet method. Further, the format of the image forming apparatus 100may be either of monochrome and color formats.

With reference to FIG. 1, the configuration and the function of theimage forming apparatus 100 are described below. As illustrated in FIG.1, the image forming apparatus 100 includes a document reading apparatus200 and an image printing apparatus 301.

<Document Reading Apparatus>

In the document reading apparatus 200, a document feeding apparatus 201is provided that feeds a document to a reading position. Documents Pstacked in a document stacking portion 2 of the document feedingapparatus 201 are fed one by one by a pickup roller 3. Then, eachdocument P is conveyed by a sheet feeding roller 4. At a positionopposed to the sheet feeding roller 4, a separation roller 5 is providedthat is in pressure contact with the sheet feeding roller 4. Theseparation roller 5 is configured to rotate if a load torque greaterthan or equal to a predetermined torque is applied to the separationroller 5. The separation roller 5 has the function of separating twodocuments fed in an overlapping state.

The pickup roller 3 and the sheet feeding roller 4 are linked togetherby a swinging arm 12. The swinging arm 12 is supported by a rotatingshaft of the sheet feeding roller 4 so that the swinging arm 12 canpivot about the rotating shaft of the sheet feeding roller 4.

The document P is conveyed by the sheet feeding roller 4 and dischargedto a sheet discharge tray 10 by sheet discharge rollers 11. Asillustrated in FIG. 1, in the document stacking portion 2, a documentset sensor SS1 is provided that detects whether the documents P arestacked in the document stacking portion 2. In a conveying path throughwhich each document P passes, a sheet sensor SS2 is provided thatdetects the front end of the document P (detects the presence or absenceof the document P).

In a reading apparatus 202, a document reading unit 16 is provided thatreads an image on a first surface of the conveyed document P. Imageinformation regarding the image read by the document reading unit 16 isoutput to the image printing apparatus 301.

In the document feeding apparatus 201, a document reading unit 17 isprovided that reads an image on a second surface of the conveyeddocument P. Image information regarding the image read by the documentreading unit 17 is output to the image printing apparatus 301 similarlyto the method of the document reading unit 16 described above.

As described above, a document is read. That is, the document feedingapparatus 201 and the reading apparatus 202 function as the documentreading apparatus 200.

The document reading apparatus 200 has a first reading mode and a secondreading mode as document reading modes. The first reading mode is a modefor reading an image on a document conveyed by the above method. Thesecond reading mode is a mode where the document reading unit 16 movingat a constant velocity reads an image on a document placed on documentglass 214 of the reading apparatus 202. Normally, an image on asheet-like document is read in the first reading mode, and an image on abound document such as a book or a booklet is read in the second readingmode.

Sheet holding trays 302 and 304 are provided in the image printingapparatus 301. In the sheet holding trays 302 and 304, different typesof recording media can be held. For example, A4-size plain paper is heldin the sheet holding tray 302, and A4-size thick paper is held in thesheet holding tray 304. On each of the recording media, an image is tobe formed by the image forming apparatus 100. For example, the recordingmedia include a sheet, a resin sheet, cloth, an overhead projector (OHP)sheet, and a label.

A recording medium held in the sheet holding tray 302 is fed by a pickuproller 303 and sent out to pre-registration rollers 333 by feedingrollers 331 and conveying rollers 306. A recording medium held in thesheet holding tray 304 is fed by a pickup roller 305 and sent out to thepre-registration rollers 333 by feeding rollers 332, conveying rollers307, and the conveying rollers 306.

Between the pre-registration rollers 333 and registration rollers 308, asheet sensor 335 for detecting the front end of the recording medium isprovided. The front end of the recording medium conveyed by thepre-registration rollers 333 is detected by the sheet sensor 335 andthen abuts the registration rollers 308 in a stopped state. Then, thepre-registration rollers 333 further rotate, thereby conveying therecording medium further in the conveying direction. Then, the recordingmedium bends. As a result, an elastic force acts on the recordingmedium, and the front end of the recording medium abuts the registrationrollers 308 along a nip portion thereof. As a result, the skew of therecording medium is corrected. In the present exemplary embodiment, thepre-registration rollers 333 are controlled to rotate for apredetermined time after the sheet sensor 335 detects the front end ofthe recording medium. The predetermined time is set in advance tosufficient time to bend the recording medium by an amount required tocorrect the skew of the recording medium.

An image signal output from the document reading apparatus 200 is inputto an optical scanning device 311 that includes a semiconductor laserand a polygon mirror. The outer peripheral surface of a photosensitivedrum 309 is charged by a charging device 310. After the outer peripheralsurface of the photosensitive drum 309 is charged, laser light accordingto the image signal input from the document reading apparatus 200 to theoptical scanning device 311 is emitted from the optical scanning device311 to the outer peripheral surface of the photosensitive drum 309 viathe polygon mirror and mirrors 312 and 313. As a result, anelectrostatic latent image is formed on the outer peripheral surface ofthe photosensitive drum 309. The photosensitive drum 309 is charged by acharging method using, for example, a corona charger or a chargingroller.

Next, the electrostatic latent image is developed with toner in adeveloping device 314, thereby forming a toner image on the outerperipheral surface of the photosensitive drum 309. The toner imageformed on the photosensitive drum 309 is transferred onto the recordingmedium by a transfer charging device 315 as a transfer unit provided ata position (a transfer position) opposed to the photosensitive drum 309.The registration rollers 308 send the recording medium to the transferposition.

The recording medium onto which the toner image has been transferred asdescribed above is sent to a fixing device 318 by a conveying belt 317and is heated and pressurized by the fixing device 318, thereby fixingthe toner image to the recording medium. In this manner, an image isformed on a recording medium by the image forming apparatus 100.

In a case where an image is formed in a one-sided printing mode, therecording medium having passed through the fixing device 318 isdischarged to a sheet discharge tray (not illustrated) by sheetdischarge rollers 319 and 324. In a case where an image is formed in atwo-sided printing mode, a fixing process is performed on a firstsurface of the recording medium by the fixing device 318, and then, therecording medium is conveyed to a reverse path 325 by the sheetdischarge rollers 319, conveying rollers 320, and reverse rollers 321.Then, the recording medium is conveyed to the registration rollers 308again by conveying rollers 322 and 323, and an image is formed on asecond surface of the recording medium by the above method. Then, therecording medium is discharged to the sheet discharge tray (notillustrated) by the sheet discharge rollers 319 and 324.

In a case where the recording medium, on the first surface of which animage is formed, is discharged face down to outside the image formingapparatus 100, the recording medium having passed through the fixingdevice 318 is conveyed through the sheet discharge rollers 319 in adirection toward the conveying rollers 320. Then, immediately before therear end of the recording medium passes through a nip portion of theconveying rollers 320, the rotation of the conveying rollers 320 isreversed, thereby discharging the recording medium to outside the imageforming apparatus 100 via the sheet discharge rollers 324 in the statewhere the first surface of the recording medium faces down.

As illustrated in FIG. 1, in the image printing apparatus 301, astacking unit 327 is provided in which a recording medium is stacked.The recording medium stacked in the stacking unit 327 is sent out in theconveying direction by a pickup roller 328 and then conveyed by sheetfeeding rollers 329.

The pickup roller 328 and one of the sheet feeding rollers 329 arelinked together by a swinging arm 330. The swinging arm 330 is supportedby the rotating shaft of the sheet feeding roller 329 so that theswinging arm 330 can pivot about the rotating shaft of the sheet feedingroller 329.

On the recording medium conveyed to the conveying rollers 306 by thesheet feeding rollers 329, an image is formed by the above method.

The configuration and the function of the image forming apparatus 100have been described above.

FIG. 2 is a block diagram illustrating an example of the controlconfiguration of the image forming apparatus 100. As illustrated in FIG.2, a system controller 151 includes a central processing unit (CPU) 151a, a read-only memory (ROM) 151 b, and a random-access memory (RAM) 151c. The system controller 151 is connected to an image processing unit112, an operation unit 152, an analog-to-digital (A/D) converter 153, ahigh voltage control unit 155, a motor control device 157, sensors 159,an alternating current (AC) driver 160, a sheet sensor 334, and thesheet sensor 335. The system controller 151 can transmit and receivedata and a command to and from the units connected to the systemcontroller 151.

The CPU 151 a reads and executes various programs stored in the ROM 151b, thereby executing various sequences related to an image formingsequence determined in advance.

The RAM 151 c is a storage device. The RAM 151 c stores various types ofdata such as a setting value for the high voltage control unit 155, aninstruction value for the motor control device 157, and informationreceived from the operation unit 152.

The system controller 151 transmits setting value data, required forimage processing by the image processing unit 112, of the variousdevices provided in the image forming apparatus 100 to the imageprocessing unit 112. Further, the system controller 151 receives signalsfrom the sensors 159, and based on the received signals, sets a settingvalue of the high voltage control unit 155. According to the settingvalue set by the system controller 151, the high voltage control unit155 supplies a required voltage to a high voltage unit 156 (the chargingdevice 310, the developing device 314, and the transfer charging device315). The sensors 159 include a sensor that detects a recording mediumconveyed by the conveying rollers.

According to an instruction output from the CPU 151 a, the motor controldevice 157 controls a motor 509 for driving the conveying rollers 307.Although only the motor 509 is illustrated as a motor of the imageforming apparatus 100 in FIG. 2, a plurality of motors is actuallyprovided in the image forming apparatus 100. Alternatively, aconfiguration may be employed in which a single motor control devicecontrols a plurality of motors. Further, although only a single motorcontrol device is provided in FIG. 2, actually, a plurality of motorcontrol devices is provided in the image forming apparatus 100.

The A/D converter 153 receives a detected signal detected by athermistor 154 that detects the temperature of a fixing heater 161.Then, the A/D converter 153 converts the detected signal from an analogsignal to a digital signal and transmits the digital signal to thesystem controller 151. The system controller 151 controls the AC driver160 based on the digital signal received from the A/D converter 153. TheAC driver 160 controls the fixing heater 161 so that the temperature ofthe fixing heater 161 becomes a temperature required to perform a fixingprocess. The fixing heater 161 is a heater for use in the fixing processand is included in the fixing device 318.

The system controller 151 controls the operation unit 152 to display, ona display unit provided in the operation unit 152, an operation screenfor a user to set the type of a recording medium to be used (hereinafterreferred to as the “paper type”). The system controller 151 receivesinformation set by the user from the operation unit 152, and based onthe information set by the user, controls the operation sequence of theimage forming apparatus 100. The system controller 151 transmits, to theoperation unit 152, information indicating the state of the imageforming apparatus 100. The information indicating the state of the imageforming apparatus 100 is, for example, information regarding the numberof images to be formed, the progress state of an image formingoperation, and a jam or multi-feed of a sheet in the document feedingapparatus 201 and the image printing apparatus 301. The operation unit152 displays on the display unit the information received from thesystem controller 151.

As described above, the system controller 151 controls the operationsequence of the image forming apparatus 100. A sheet detector 700 willbe described below.

[Motor Control Device]

Next, the motor control device according to the present exemplaryembodiment is described. The motor control device according to thepresent exemplary embodiment controls a motor using vector control.

<Vector Control>

First, with reference to FIGS. 3 and 4, a description is given of amethod in which the motor control device 157 performs vector control,according to the present exemplary embodiment. In a motor in the Wowingdescription, a sensor such as a rotary encoder for detecting therotational phase of a rotor of the motor is not provided. Alternatively,a sensor such as a rotary encoder may be provided.

FIG. 3 is a diagram illustrating the relationship between the steppermotor (hereinafter referred to as “motor”) 509 that has two phasesincluding an A-phase (a first phase) and a B-phase (a second phase), anda rotating coordinate system represented by a d-axis and a q-axis. InFIG. 3, in a stationary coordinate system, an α-axis, which is an axiscorresponding to windings in the A-phase, and a β-axis, which is an axiscorresponding to windings in the B-phase, are defined. In FIG. 3, thed-axis is defined along the direction of magnetic flux created by themagnetic poles of a permanent magnet used in a rotor 402, and the q-axisis defined along a direction rotated 90 degrees counterclockwise fromthe d-axis (a direction orthogonal to the d-axis). The angle between theα-axis and the d-axis is defined as θ, and the rotational phase of therotor 402 is represented by the angle θ. In the vector control, arotating coordinate system based on the rotational phase θ of the rotor402 is used. Specifically, in the vector control, a q-axis component (atorque current component) and a d-axis component (an excitation currentcomponent), which are current components in the rotating coordinatesystem of a current vector corresponding to a driving current flowingthrough each winding, are used. The q-axis component (the torque currentcomponent) generates a torque in the rotor 402, and the d-axis component(the excitation current component) influences the strength of magneticflux passing through the winding.

The vector control is a control method for controlling a motor byperforming phase feedback control for controlling the value of a torquecurrent component and the value of an excitation current component sothat the deviation between an instruction phase indicating a targetphase of a rotor and an actual rotational phase of the rotor becomessmall. There is also a method for controlling a motor by performingvelocity feedback control for controlling the value of a torque currentcomponent and the value of an excitation current component so that thedeviation between an instruction velocity indicating a target velocityof a rotor and an actual rotational velocity of the rotor becomes small.

FIG. 4 is a block diagram illustrating an example of the configurationof the motor control device 157 that controls the motor 509. The motorcontrol device 157 includes at least one ASIC and executes functionsdescribed below.

The motor control device 157 includes, as a circuit for performing thevector control, a phase controller 502, a current controller 503, acoordinate inverse transformer 505, a coordinate transformer 511, and apulse-width modulation (PWM) inverter 506 that supplies driving currentsto the windings of the motor 509. The coordinate transformer 511performs coordinate transformation on current vectors corresponding todriving currents flowing through the windings in the A-phase and theB-phase of the motor 509, from the stationary coordinate systemrepresented by the α-axis and the β-axis to the rotating coordinatesystem represented by the q-axis and the d-axis. As a result, thedriving currents flowing through the windings are represented by thecurrent value of the q-axis component (a q-axis current) and the currentvalue of the d-axis component (a d-axis current), which are currentvalues in the rotating coordinate system. The q-axis current correspondsto a torque current that generates a torque in the rotor 402 of themotor 509. The d-axis current corresponds to an excitation current thatinfluences the strength of magnetic flux passing through each winding ofthe motor 509. The motor control device 157 can independently controlthe q-axis current and the d-axis current. As a result, the motorcontrol device 157 controls the q-axis current according to a loadtorque applied to the rotor 402 and thereby can efficiently generate atorque required for the rotation of the rotor 402. That is, in thevector control, the magnitude of the current vector illustrated in FIG.3 changes according to the load torque applied to the rotor 402.

The motor control device 157 determines the rotational phase θ of therotor 402 of the motor 509 using a method described below, and based onthe determination result, performs the vector control, Based on theoperation sequence of the motor 509, the CPU 151 a outputs, to aninstruction generator 500, driving pulses as an instruction to drive themotor 509. The operation sequence of the motor 509 (the driving patternof the motor 509) is stored, for example, in the ROM 151 b. Based on theoperation sequence stored in the ROM 151 b, the CPU 151 a outputsdriving pulses as a pulse train. The number of pulses corresponds to aninstruction phase, and the frequency of pulses corresponds to a targetvelocity.

Based on the driving pulses output from the CPU 151 a, the instructiongenerator 500 generates an instruction phase θ_ref representing a targetphase of the rotor 402 and outputs the instruction phase θ_ref. Theconfiguration of the instruction generator 500 will be described below.

A subtractor 101 calculates a deviation AO between the rotational phaseθ of the rotor 402 of the motor 509 and the instruction phase θ_ref andoutputs the deviation Δθ.

The phase controller 502 acquires the deviation Δθ in a period T (e.g.,200 μs). Based on proportional control (P-control), integral control(I-control), and differential control (D-control), the phase controller502 generates a q-axis current instruction value iq_ref and a d-axiscurrent instruction value id_ref as target values so that the deviationΔθ output from the subtractor 101 becomes small. Then, the phasecontroller 502 outputs the q-axis current instruction value iq_ref andthe d-axis current instruction value id_ref. Specifically, based on theP-control, the I-control, and the D-control, the phase controller 502generates the q-axis current instruction value iq_ref and the d-axiscurrent instruction value id_ref so that the deviation Δθ output fromthe subtractor 101 becomes 0. Then, the phase controller 502 outputs theq-axis current instruction value iq_ref and the d-axis currentinstruction value id_ref. The P-control is a control method forcontrolling the value of a target to be controlled, based on a valueproportional to the deviation between an instruction value and anestimated value. The I-control is a control method for controlling thevalue of the target to be controlled, based on a value proportional tothe time integral of the deviation between the instruction value and theestimated value. The D-control is a control method for controlling thevalue of the target to be controlled, based on a value proportional to achange over time in the deviation between the instruction value and theestimated value. The phase controller 502 according to the presentexemplary embodiment generates the q-axis current instruction valueiq_ref and the d-axis current instruction value id_ref based onproportional-integral-derivative (PID) control. The configuration,however, is not limited to this. For example, the phase controller 502may generate the q-axis current instruction value iq_ref and the d-axiscurrent instruction value id_ref based on proportional-integral (PI)control. In a case where a permanent magnet is used in the rotor 402,normally, the d-axis current instruction value id_ref, which influencesthe strength of magnetic flux passing through each winding, is set to 0.The configuration, however, is not limited to this.

A driving current flowing through the windings in the A-phase of themotor 509 is detected by a current detector 507 and then converted froman analog value to a digital value by an A/D converter 510. A drivingcurrent flowing through the windings in the B-phase of the motor 509 isdetected by a current detector 508 and then converted from an analogvalue to a digital value by the A/D converter 510. The cycle in whichthe current detectors 507 and 508 detect the currents is, for example, acycle (e.g., 25 μs) less than or equal to the cycle T, in which thephase controller 502 acquires the deviation Δθ.

The current values of the driving currents converted from the analogvalues to the digital values by the A/D converter 510 are represented ascurrent values iα and iα in the stationary coordinate system by thefollowing formulas (1) and (2), using a phase θe of the current vectorillustrated in FIG. 3. The phase θe of the current vector is defined asthe angle between the α-axis and the current vector. I represents themagnitude of the current vector.

iα=I*cos θe  (1)

iβ=I*sin θe  (2)

The current values iα and iβ are input to the coordinate transformer 511and an inductive voltage determiner 512.

The coordinate transformer 511 transforms the current values iα and iβin the stationary coordinate system into a current value iq of theq-axis current and a current value id of the d-axis current in therotating coordinate system by the following formulas (3) and (4).

id=cos θ*iα+sin θ*iβ  (3)

iq=−sin θ*iα+cos θ*iβ  (4)

The coordinate transformer 511 outputs the transformed current value iqto a subtractor 102. The coordinate transformer 511 outputs thetransformed current value id to a subtractor 103.

The subtractor 102 calculates the deviation between the q-axis currentinstruction value iq_ref and the current value iq and outputs thecalculated deviation to the current controller 503.

The subtractor 103 calculates the deviation between the d-axis currentinstruction value id_ref and the current value id and outputs thecalculated deviation to the current controller 503.

Based on the PID control, the current controller 503 generates drivingvoltages Vq and Vd so that each of the deviations input to the currentcontroller 503 becomes small. Specifically, the current controller 503generates the driving voltages Vq and Vd so that each of the deviationsinput to the current controller 503 becomes 0. Then, the currentcontroller 503 outputs the driving voltages Vq and Vd to the coordinateinverse transformer 505. That is, the current controller 503 functionsas a generation unit. The current controller 503 according to thepresent exemplary embodiment generates the driving voltages Vq and Vdbased on the PID control. The configuration, however, is not limited tothis. For example, the current controller 503 may generate the drivingvoltages Vq and Vd based on the PI control.

The coordinate inverse transformer 505 inversely transforms the drivingvoltages Vq and Vd in the rotating coordinate system, which are outputfrom the current controller 503, into driving voltages Vα and Vβ in thestationary coordinate system by the following formulas (5) and (6).

Vα=cos θ*Vd−sin θ*Vq  (5)

Vβ=sin θ*Vd+cos θ*Vq  (6)

The coordinate inverse transformer 505 outputs the inversely transformeddriving voltages Vα and Vβ to the inductive voltage determiner 512 andthe PWM inverter 506.

The PWM inverter 506 includes a full-bridge circuit. The full-bridgecircuit is driven by PWM signals based on the driving voltages Vα and Vβinput from the coordinate inverse transformer 505. As a result, the PWMinverter 506 generates driving currents iα and iβ according to thedriving voltages Vα and Vβ and supplies the driving currents iα and iβto the windings in the respective phases of the motor 509, therebydriving the motor 509. In the present exemplary embodiment, the PWMinverter 506 includes a full-bridge circuit. Alternatively, the PWMinverter 506 may include a half-bridge circuit.

Next, a description is given of a configuration for determining therotational phase θ. The rotational phase θ of the rotor 402 isdetermined using the values of inductive voltages Eα and Eβ induced inthe windings in the A-phase and the B-phase of the motor 509 by therotation of the rotor 402. The value of each inductive voltage isdetermined (calculated) by the inductive voltage determiner 512.Specifically, the inductive voltages Eα and Eβ are determined by thefollowing formulas (7) and (8), based on the current values iα and iβinput from the A/D converter 510 to the inductive voltage determiner 512and the driving voltages Vα and Vβ input from the coordinate inversetransformer 505 to the inductive voltage determiner 512.

Eα=Vα−R*iα−L*diα/dt  (7)

Eβ=Vβ−R*iβ−L*diβ/dt  (8)

In these formulas, R represents winding resistance, and L representswinding inductance. The values of the winding resistance R and thewinding inductance L are values specific to the motor 509 in use and arestored in advance in the ROM 151 b or a memory (not illustrated)provided in the motor control device 157.

The inductive voltages Eα and Eβ determined by the inductive voltagedeterminer 512 are output to the phase determiner 513.

Based on the ratio between the inductive voltages Eα and Eβ output fromthe inductive voltage determiner 512, the phase determiner 513determines the rotational phase θ of the rotor 402 of the motor 509 bythe following formula (9).

θ=tan {circumflex over ( )}−1(−Eβ/Eα)  (9)

In the present exemplary embodiment, the phase determiner 513 determinesthe rotational phase θ by performing calculation based on formula (9).The configuration, however, is not limited to this. For example, thephase determiner 513 may determine the rotational phase θ by referencinga table stored in a memory 513 a and illustrating the relationshipsbetween the inductive voltages Eα and Eβ, and the rotational phase θcorresponding to the inductive voltages Eα and Eβ.

The rotational phase θ of the rotor 402 obtained as described above isinput to the subtractor 101, the coordinate inverse transformer 505, andthe coordinate transformer 511.

The motor control device 157 repeatedly performs the above control.

As described above, the motor control device 157 according to thepresent exemplary embodiment performs the vector control using phasefeedback control for controlling current values in the rotatingcoordinate system so that the deviation Δθ between the instruction phaseθ_ref and the rotational phase θ becomes small. The vector control isperformed, whereby it is possible to prevent a motor from entering astep-out state and prevent an increase in the motor sound and anincrease in power consumption due to an excess torque.

<Instruction Generator>

Based on the driving pulses output from the CPU 151 a, the instructiongenerator 500 generates the instruction phase θ_ref using the followingformula (10) and outputs the instruction phase θ_ref.

θ_ref=θini+θstep*n  (10)

θini is the phase (initial phase) of the rotor 402 when the driving ofthe motor 509 is started. θstep is the amount of increase (the amount ofchange) in the instruction phase θ_ref per driving pulse. n is thenumber of pulses input to the instruction generator 500.

[Control of Conveyance of Sheet in Image Forming Apparatus] <SheetDetector>

FIG. 5 is a diagram illustrating a configuration for detecting a fedrecording medium. As illustrated in FIG. 5, the conveying rollers 307are driven by the motor 509, and the motor 509 is controlled by themotor control device 157. The feeding rollers 332 and the pickup roller305 are driven by motors (not illustrated). The feeding rollers 332 arerollers adjacent to the conveying rollers 307. In the present exemplaryembodiment, a conveying velocity V at which a recording medium isconveyed is set to a predetermined velocity V0 in advance based on theoperation sequence of the image forming apparatus 100.

Next, a description is given of a configuration in which the sheetdetector 700 detects whether the front end of the recording mediumreaches a nip portion of the conveying rollers 307. In the presentexemplary embodiment, it is detected (determined) whether the front endof the recording medium reaches the nip portion of the conveying rollers307 not by a sensor such as a photosensor but based on a signal outputfrom the motor control device 157. In the following description, forexample, the sheet detector 700 outputs the detection result in apredetermined time cycle (e.g., the cycle in which the deviation Δθ isinput).

The front end of the recording medium conveyed downstream by the feedingrollers 332 is nipped by the conveying rollers 307. If the front end ofthe recording medium is nipped by the conveying rollers 307, the loadtorque applied to the rotor 402 of the motor 509 for driving theconveying rollers 307 increases. If the load torque increases, theabsolute value of the deviation Δθ increases.

If the absolute value of the deviation Δθ becomes greater than or equalto a threshold Δθth as a predetermined value, the sheet detector 700outputs a signal ‘1’ indicating that the absolute value of the deviationΔθ becomes greater than or equal to the threshold Δθth (the recordingmedium is detected). If the absolute value of the deviation Δθ is lessthan the threshold Δθth, the sheet detector 700 outputs a signal ‘0’indicating that the absolute value of the deviation Δθ is less than thethreshold Δθth. The threshold Δθth will be described below.

<Adjustment of Conveying Velocity V>

The detection result of the sheet detector 700 is input to the CPU 151a. If the sheet detector 700 outputs the signal ‘1’, the CPU 151 aadjusts the conveying velocity V of the recording medium. For example,the CPU 151 a changes the frequencies of driving pulses to be output tomotor control devices provided in the image forming apparatus 100,thereby adjusting the conveying velocity V.

In the following description, X1 represents the distance from the pickuproller 305 to the conveying rollers 307. X2 represents the distance fromthe conveying rollers 307 to a detection position where the sheet sensor334 detects the recording medium. That is, the distance from the pickuproller 305 to the detection position is represented by X1+X2. T0corresponds to time required for the recording medium to be conveyed bythe distance X1+X2 at the conveying velocity V0.

In the present exemplary embodiment, the pickup roller 305 is repeatedlyrotated and stopped at predetermined time intervals, thereby feedingrecording media at predetermined intervals. As illustrated in FIG. 2,the CPU 151 a includes a timer 151 d and measures the time elapsed sincethe driving of the pickup roller 305 is started (since the CPU 151 aoutputs an instruction to start driving the pickup roller 305).

The CPU 151 a sets as the conveying velocity V a velocity calculatedbased on the distance X2 and time period obtained by subtracting fromtime period T0 time period Ta, the time period Ta being a period fromwhen the driving of the pickup roller 305 is started to when the sheetdetector 700 outputs the signal ‘1’. Specifically, based on thefollowing formula (11), the CPU 151 a sets the conveying velocity V in asection from the conveying rollers 307 to the detection position (i.e.,the peripheral velocity of conveying rollers in the section from theconveying rollers 307 to the detection position). The conveying velocityV in a section from the pickup roller 305 to the conveying rollers 307after the conveying velocity V in the section from the conveying rollers307 to the detection position is adjusted may be set to V0, or may beset to the conveying velocity V adjusted based on formula (11).

V=X2/(T0−Ta)  (11)

FIG. 6 is a diagram illustrating the deviation Δθ output from the motorcontrol device 157 in a case where thin paper is conveyed (a dashedline), and the deviation Δθ output from the motor control device 157 ina case where thick paper is conveyed (a solid. line). In FIG. 6, thetiming when a feeding operation for feeding the recording medium isstarted is illustrated as t=0.

In FIG. 6, the deviation Δθ having a positive value means that therotational phase θ is behind the instruction phase θ_ref. The deviationΔθ having a negative value means that the rotational phase θ is ahead ofthe instruction phase θ_ref. However, the relationships between thepolarity of the deviation Δθ, and the rotational phase θ and theinstruction phase θ_ref are not limited to these. For example, aconfiguration may be employed in which, in a case where the rotationalphase θ is behind the instruction phase θ_ref, the deviation Δθ has anegative value, and in a case where the rotational phase θ is ahead ofthe instruction phase θ_ref, the deviation Δθ has a positive value. Asillustrated in FIG. 6, if the load torque increases, the absolute valueof the deviation Δθ becomes great due to the fact that the rotationalphase θ of the rotor 402 of the motor 509 is behind the instructionphase θ_ref.

FIGS. 7A and 7B are diagrams illustrating the position of the front endof the recording medium at the timing when the sheet detector 700outputs the signal ‘1’ (the timing when the recording medium isdetected).

FIG. 7A is a diagram illustrating the position of the front end of thethin paper at the timing (a time ta) when the sheet detector 700 outputsthe signal ‘1’ in a case where the thin paper is conveyed. FIG. 7B is adiagram illustrating the position of the front end of the thick paper atthe timing (a time tb) when the sheet detector 700 outputs the signal‘1’ in a case where the thick paper is conveyed.

The front end of the recording medium conveyed downstream by the feedingrollers 332 is nipped by the conveying rollers 307. If the front end ofthe recording medium is nipped by the conveying rollers 307, the loadtorque applied to the rotor 402 of the motor 509 for driving theconveying rollers 307 increases. If the load torque increases, theabsolute value of the deviation Δθ increases, for example, asillustrated in FIG. 6 (the time ta or tb).

In the present exemplary embodiment, the conveying rollers 307 rotate ata peripheral velocity faster than the peripheral velocity of the feedingrollers 332. If the recording medium is nipped by the conveying rollers307, the conveying rollers 307 pull the recording medium nipped by thefeeding rollers 332 downstream. With such a configuration, it ispossible to make the range of increase in the load torque when therecording medium is nipped by the conveying rollers 307 greater. Thus,the front end of the recording medium is detected with higher accuracy.

In the present exemplary embodiment, the threshold Δθth is set to, forexample, a value smaller than the load torque applied to the conveyingrollers 307 that increases due to a recording medium having the smallestrigidity and thickness among a plurality of types of recording mediathat can be conveyed in the image forming apparatus 100, i.e., a valuesmaller than the maximum value (the peak value) of the absolute value ofthe deviation Δθ. Further, the threshold Δθth is set to, for example, avalue smaller than the load torque applied to the conveying rollers 307that increases due to a recording medium having the greatest rigidityand thickness among the plurality of types of recording media that canbe conveyed in the image forming apparatus 100, i.e., a value smallerthan the maximum value (the peak value) of the absolute value of thedeviation Δθ.

The threshold Δθth is set to, for example, a value greater than theabsolute value of the deviation Δθ assumed in the state where therecording medium is not nipped by the nip portion of the conveyingrollers 307 and also the state where the conveying rollers 307 rotate ata constant velocity.

As illustrated in FIGS. 7A and 7B, the front end of the recording mediumis nipped at the timing when the front end of the recording medium islocated upstream of a nip position n in the conveying direction due tothe thickness of the recording medium. As illustrated in FIGS. 7A and7B, a distance La from the position of the front end of the thin paperat the timing when the sheet detector 700 outputs the signal ‘1’ to thenip position n in a case where the thin paper is conveyed is shorterthan a distance Lb from the position of the front end of the thick paperat the timing when the sheet detector 700 outputs the signal ‘1’ to thenip position n in a case where the thick paper is conveyed. This meansthat, due to the fact that the thickness of the thick paper is greaterthan the thickness of the thin paper, the position of the front end ofthe thick paper when the conveying rollers 307 start nipping the thickpaper is located upstream of the position of the front end of the thinpaper when the conveying rollers 307 start nipping the thin paper.

As described above, a distance Y from the position of the front end ofthe recording medium at the timing when the sheet detector 700 outputsthe signal ‘1’ to the detection position is different from the distanceX2. In other words, the distance Y is longer than the distance X2. Ifthe conveying velocity V is set based on formula (11), the position ofthe front end of the recording medium at the timing when the recordingmedium is to reach the detection position is located upstream of thedetection position due to the fact that the distance Y is longer thanthe distance X2. That is, the recording medium may reach the detectionposition after the timing when the recording medium is to reach thedetection position. As a result, the recording medium may reach thetransfer position after the timing when the transfer of an image ontothe recording medium is started, and the image may not be formed at anappropriate position on the recording medium.

In the present exemplary embodiment, the following configuration isapplied, thereby preventing the situation where an image is formed at aninappropriate position on a recording medium.

FIG. 8 is a diagram illustrating the relationship between the grammageof the recording medium to be conveyed, and a distance Lc from theposition of the front end of the recording medium at the timing when thesheet detector 700 outputs the signal ‘1’ to the conveying rollers 307.A grammage Ma in FIG. 8 corresponds to the grammage of the thin paper. Agrammage Mb in FIG. 8 corresponds to the grammage of the thick paper.The relationship between the grammage and the distance Lc illustrated inFIG. 8 is obtained by experiment and stored in advance, for example, inthe ROM 151 b.

A distance Lc_a is a value corresponding to La illustrated in FIG. 7A. Adistance Lc_b is a value corresponding to Lb illustrated in FIG. 7B.

Information regarding the paper type is, for example, input by the userthrough the operation unit 152. The information regarding the paper typeincludes the grammage and the rigidity of the recording medium. Based onthe input information regarding the paper type, and the relationshipbetween the grammage and the distance Lc stored in the ROM 151 b, theCPU 151 a determines the distance Lc. For example, if informationindicating that the thin paper is to be conveyed is input by the userthrough the operation unit 152, the CPU 151 a sets Lc_a corresponding tothe thin paper as the distance Lc. If information indicating that thethick paper is to be conveyed is input by the user through the operationunit 152, the CPU 151 a sets Lc_b corresponding to the thick paper asthe distance Lc.

Using the set distance Lc, the CPU 151 a sets the conveying velocity Vbased on the following formula (12).

V=(X2+Lc)/(T0−Ta)  (12)

That is, the CPU 151 a calculates the distance from the position of thefront end of the recording medium at e timing when the sheet detector700 outputs the signal ‘1’ to the registration rollers 308. Then, theCPU 151 a sets the conveying velocity V by dividing the calculateddistance by a value obtained by subtracting the time period Ta from thetime period T0. That is, the CPU 151 a sets the conveying velocity Vbased on the position of the front end of the recording medium at thetiming when the sheet detector 700 outputs the signal ‘1’.

FIG. 9 is a flowchart illustrating a control method for controlling theconveying velocity V by the CPU 151 a. With reference to FIG. 9, controlof the conveying velocity V according to the present exemplaryembodiment is described. The processing of the flowchart is executed bythe CPU 151 a. During the processing of the flowchart, the CPU 151 aresets and starts the timer 151 d each time the CPU 151 a outputs aninstruction to start rotationally driving the pickup roller 305.

In step S1001, if information regarding the paper type is input to theCPU 151 a through the operation unit 152 (YES in step S1001), then instep S1002, the CPU 151 a sets the distance Lc based on the inputinformation regarding the paper type.

Then, in step S1003, the CPU 151 a starts a feeding operation forfeeding a recording medium stored in a specified sheet holding tray.From this point forward, the pickup roller 305 is repeatedly driven andstopped at predetermined time intervals.

Next, in step S1004, CPU 151 a determines whether the sheet detector 700outputs the signal ‘1’. If the sheet detector 700 outputs the signal ‘1’(YES in step S1004), the processing proceeds to step S1005.

In step S1005, the CPU 151 a adjusts (sets) the conveying velocity Vbased on the distance Lc set in step S1002, the time period Ta from whenthe driving of the pickup roller is started to when the sheet detector700 outputs the signal ‘1’, and the distance X2. Specifically, the CPU151 a sets the conveying velocity V using formula (12).

In step S1006, the CPU 151 a determines whether a print job is to beended. If a print job is to be ended (YES in step S1006), then in stepS1007, the CPU 151 a ends the feeding operation.

On the other hand, in step S1006, if the print job is not to be ended(NO in step S1006), the processing returns to step S1004.

In step S1004, if the sheet detector 700 does not output the signal ‘1’(NO in step S1004), the processing proceeds to step S1008.

In step S1008, the CPU 151 a determines whether the state where thesheet detector 700 does not output the signal ‘1’ continues for apredetermined time. If the state where the sheet detector 700 does notoutput the signal ‘1’ does not continue for the predetermined time (NOin step S1008), the processing returns to step S1004.

On the other hand, in step S1008, if the state where the sheet detector700 does not output the signal ‘1’ continues for the predetermined time(YES in step S1008), then in step S1009, the CPU 151 a stops the feedingoperation. The predetermined time is set to, for example, time longerthan time required for the recording medium fed by the pickup roller 305to be conveyed at the conveying velocity V0 and reach the conveyingrollers 307.

Then, in step S1010, the CPU 151 a notifies the user that an abnormality(e.g., a jam) occurs in the conveyance of the recording medium, bydisplaying the notification on the display unit provided in theoperation unit 152.

As described above, in the present exemplary embodiment, based on thedistance X2 and the distance Lc that occurs due to the thickness of therecording medium, the distance from the position of the front end of therecording medium at the timing when the sheet detector 700 outputs thesignal ‘1’ to the registration rollers 308 is calculated. The conveyingvelocity V is set by dividing the calculated distance by a valueobtained by subtracting the time period Ta from the time period T0. Thatis, in the present exemplary embodiment, the conveying velocity V is setbased on the position of the front end of the recording medium at thetiming when the sheet detector 700 outputs the signal ‘1’. As a result,it is possible to prevent a recording medium from reaching a transferposition after the timing when the transfer of an image onto therecording medium is started. Thus, it is possible to prevent thesituation where the image is formed at an inappropriate position on therecording medium.

Further, in the present exemplary embodiment, the conveying velocity Vis set based on the distance Lc corresponding to the paper type. As aresult, it is possible to prevent a recording medium from reaching atransfer position after the timing when the transfer of an image ontothe recording medium is started due to the fact that the position of thefront end of the recording medium at the timing when the sheet detector700 outputs the signal ‘1’ differs depending on the paper type. That is,it is possible to prevent the situation where the image is not formed atan appropriate position on the recording medium.

A second exemplary embodiment is described below. Components of theimage forming apparatus 100 similar to those according to the firstexemplary embodiment are not described here.

In the first exemplary embodiment, the CPU 151 a sets the conveyingvelocity V based on the distance from the position of the front end ofthe recording medium at the timing when the sheet detector 700 outputsthe signal ‘1’ to the registration rollers 308. In the present exemplaryembodiment, the CPU 151 a sets the conveying velocity V based on thetiming when the front end of the recording medium reaches a nip positionof conveying rollers.

FIG. 10 is a diagram illustrating the relationship between the grammageof the recording medium to be conveyed, and time Tc from when therecording medium is detected to when the front end of the recordingmedium reaches the nip position n. A grammage Ma in FIG. 10 correspondsto the grammage of the thin paper. A grammage Mb in FIG. 10 correspondsto the grammage of the thick paper. The relationship between thegrammage and the time Tc illustrated in FIG. 10 is obtained byexperiment and stored in advance, for example, in the ROM 151 b.

Time Tc_a and time Tc_b are values obtained by dividing by the conveyingvelocity V0 the distance from the position of the front end of therecording medium at the timing when the sheet detector 700 outputs thesignal ‘1’ to the nip position n of the conveying rollers 307.Specifically, the time Tc_a and the time Tc_b are represented by thefollowing formulas (13) and (14).

Tc_a=La/V0  (13)

Tc_b=Lb/V0  (14)

Information regarding the type of the recording medium (the paper type)specified by the user via the operation unit 152 is input to the CPU 151a. Based on the acquired information regarding the paper type, and therelationship between the grammage and the time Tc stored in the ROM 151b, the CPU 151 a determines the time Tc. For example, if informationindicating that the thin paper is to be conveyed is input by the uservia the operation unit 152, the CPU 151 a sets the time Tc_acorresponding to the thin paper as the time Tc. If informationindicating that the thick paper is to be conveyed is input by the uservia the operation unit 152, the CPU 151 a sets the time Tc_bcorresponding to the thick paper as the time Tc.

Using the set time Tc, the CPU 151 a sets the conveying velocity V basedon the following formula (15). Specifically, the CPU 151 a sets theconveying velocity V in the section from the conveying rollers 307 tothe detection position (i.e., the peripheral velocity of conveyingrollers in the section from the conveying rollers 307 to the detectionposition). The conveying velocity V in the section from the pickuproller 305 to the conveying rollers 307 after the conveying velocity Vin the section from the conveying rollers 307 to the detection positionis adjusted may be set to V0, or may be set to the adjusted conveyingvelocity V.

V=X2/(T0−(Ta+Tc))  (15)

That is, based on the time Ta and the time Tc, the CPU 151 a calculatesthe time from when the driving of the pickup roller 305 is started towhen the front end of the recording medium reaches the nip position n ofthe conveying rollers 307. Then, the CPU 151 a sets the conveyingvelocity V by dividing the distance X2 by a value obtained bysubtracting the calculated time from the time T0. That is, the CPU 151 asets the conveying velocity V based on the timing when the front end ofthe recording medium actually reaches a nip position of conveyingrollers.

As described above, in the present exemplary embodiment, based on thetime Ta and the time Tc, the time from when the driving of the pickuproller 305 is started to when the front end of the recording mediumreaches the nip position n of the conveying rollers 307 is calculated.The conveying velocity V is set by dividing the distance X2 by a valueobtained by subtracting the calculated time from the time T0. That is,in the present exemplary embodiment, the conveying velocity V is setbased on the timing when the front end of the recording medium reaches anip position of conveying rollers. As a result, it is possible toprevent a recording medium from reaching a transfer position after thetiming when the transfer of an image onto the recording medium isstarted. Thus, it is possible to prevent the situation where the imageis formed at an inappropriate position on the recording medium.

Further, in the present exemplary embodiment, the conveying velocity Vis set based on the time Tc corresponding to the paper type. As aresult, it is possible to prevent a recording medium from reaching atransfer position after the timing when the transfer of an image ontothe recording medium is started due to the fact that the position of thefront end of the recording medium at the timing when the sheet detector700 outputs the signal ‘1’ differs depending on the paper type. That is,it is possible to prevent the situation where the image is formed at aninappropriate position on the recording medium.

A third exemplary embodiment is described below. Components of the imageforming apparatus 100 similar to those according to the first exemplaryembodiment are not described here.

FIGS. 11A and 11B are diagrams illustrating the position of the frontend of the recording medium at the timing when the sheet detector 700outputs the signal ‘1’, according to the present exemplary embodiment.FIG. 11A is a diagram illustrating the position of the front end of thethin paper at the timing when the sheet detector 700 outputs the signal‘1’ in a case where the thin paper is conveyed. FIG. 11B is a diagramillustrating the position of the front end of the thick paper at thetiming when the sheet detector 700 outputs the signal ‘1’ in a casewhere the thick paper is conveyed.

In the present exemplary embodiment, the conveying path from the feedingrollers 332 to the conveying rollers 307 is curved. Thus, the front endof the recording medium conveyed downstream by the feeding rollers 332collides with the conveying rollers 307 and then is guided to the nipposition n of the conveying rollers 307. Then, the front end of therecording medium is nipped by the conveying rollers 307.

In a case where thin paper as a recording medium having small rigidity(or thickness) is conveyed, the amount of increase in the load torqueapplied to the rotor 402 of the motor 509 that occurs when the front endof the thin paper collides with the conveying rollers 307 is relativelysmall. On the other hand, the amount of increase in the load torqueapplied to the rotor 402 of the motor 509 that occurs due to the factthat the front end of the thin paper is nipped by the conveying rollers307 is greater than the amount of increase in the load torque thatoccurs when the front end of the thin paper collides with the conveyingrollers 307.

The amount of increase in the load torque that occurs when the front endof thick paper having greater rigidity and thickness than those of thethin paper collides with the conveying rollers 307 is greater than theamount of increase in the load torque that occurs when the front end ofthe thin paper collides with the conveying rollers 307.

FIG. 12 is a diagram illustrating the state of the deviation Δθaccording to the present exemplary embodiment. As indicated by adashed-dotted line in FIG. 12, the absolute value of the deviation Δθincreases at the timing (a time ta) when the front end of the thin papercollides with the conveying rollers 307. As indicated by a solid line inFIG. 12, the absolute value of the deviation Δθ increases at the timing(a time tc) when the front end of the thick paper collides with theconveying rollers 307 and the timing (a time tb) when the front end ofthe thick paper is nipped by the conveying rollers 307.

As illustrated in FIG. 12, in a case where the thick paper is conveyed,at a timing (the time tc) before the timing (the time tb) when the thickpaper is nipped by the conveying rollers 307, the deviation Δθ increasesdue to the fact that the thick paper collides with the conveying rollers307. That is, a distance Lb′ has a value greater than that of thedistance Lb in the first exemplary embodiment.

The distance Y from the position of the front end of the recordingmedium at the timing when the sheet detector 700 outputs the signal ‘1’to the detection position is different from the distance X2.Specifically, the distance Y is longer than the distance X2. if theconveying velocity V is set based on formula (11), the position of thefront end of the recording medium at the timing when the recordingmedium is to reach the detection position is located upstream of thedetection position due to the fact that the distance Y is longer thanthe distance X2. In other words, the recording medium may reach thedetection position after the timing when the recording medium is toreach the detection position. As a result, the recording medium mayreach the transfer position after the timing when the transfer of animage onto the recording medium is started, and the image may not beformed at an appropriate position on the recording medium.

In response, in the present exemplary embodiment, the followingconfiguration is applied, thereby preventing the situation where animage is formed at an inappropriate position on a recording medium.

FIG. 13 is a diagram illustrating the relationship between the grammageof the recording medium to be conveyed, and a distance Lc′ from theposition of the front end of the recording medium at the timing when thesheet detector 700 outputs the signal ‘1’ to the conveying rollers 307.A grammage Mb′ corresponds to, for example, the smallest grammage amongthe grammages of the recording medium for which the sheet detector 700outputs the signal ‘1’ due to the fact that the recording mediumcollides with the conveying rollers 307. The relationship between thegrammage and the distance Lc′ illustrated in FIG. 13 is obtained byexperiment and stored in advance, for example, in the ROM 151 b.

A distance L1 is a value corresponding to La illustrated in FIG. 11A. Adistance L2 is a value corresponding to Lb′ illustrated in FIG. 11B.

The CPU 151 a determines the distance Lc′ based on acquired informationregarding the paper type, and the relationship between grammage and thedistance Lc′ stored in the ROM 151 b.

For example, a recording medium having a grammage greater than or equalto the grammage Mb′ is detected by the sheet detector 700 due to thefact that the recording medium collides with the conveying rollers 307.At this time, the timing when the recording medium fed by the pickuproller 305 collides with the conveying rollers 307 is approximately thesame regardless of the paper type. Thus, the distance from the positionof the front end of the recording medium having a grammage greater thanor equal to the grammage Mb′ at the timing when the recording medium isdetected by the sheet detector 700 to the nip position n isapproximately the same (Lb′) regardless of the paper type. Thus, in thepresent exemplary embodiment, if the grammage input by the user via theoperation unit 152 is greater than or equal to Mb′, the CPU 151 a setsL2 as the distance Lc′.

On the other hand, if the grammage input by the user via the operationunit 152 is less than or equal to Mb′, the CPU 151 a sets the distanceLc′ according to information regarding the input grammage.

Using the set distance Lc′, the CPU 151 a sets the conveying velocity Vbased on formula (12).

That is, the CPU 151 a calculates the distance from the position of thefront end of the recording medium at the timing when the sheet detector700 outputs the signal ‘1’ to the registration rollers 308. Then, theCPU 151 a sets the conveying velocity V by dividing the calculateddistance by a value obtained by subtracting the time Ta from the timeT0. That is, the CPU 151 a sets the conveying velocity V based on theposition of the front end of the recording medium at the timing when thesheet detector 700 outputs the signal ‘1’.

As described above, in the present exemplary embodiment, based on thedistance X2 and the distance Lc′ at the timing when the recording mediumis detected by the sheet detector 700 due to the fact that the recordingmedium collides with the conveying rollers 307, the distance from theposition of the front end of the recording medium at this timing to thedetection position is calculated. The conveying velocity V is set bydividing the calculated distance by a value obtained by subtracting thetime Ta from the time T0. That is, in the present exemplary embodiment,the conveying velocity V is set based on the position of the front endof the recording medium at the timing when the sheet detector 700outputs the signal ‘1’. As a result, it is possible to prevent arecording medium from reaching a transfer position after the timing whenthe transfer of an image onto the recording medium is started. Thus, itis possible to prevent the situation where the image is formed at aninappropriate position on the recording medium.

Further, in the present exemplary embodiment, the conveying velocity Vis set based on the distance Lc′ corresponding to the paper type. As aresult, it is possible to prevent a recording medium from reaching atransfer position after the timing when the transfer of an image ontothe recording medium is started due to the fact that the position of thefront end of the recording medium at the timing when the sheet detector700 outputs the signal ‘1’ differs depending on the paper type. That is,it is possible to prevent the situation where the image is formed at aninappropriate position on the recording medium.

Alternatively, the conveying velocity V may be adjusted by the methodaccording to the second exemplary embodiment based on the position ofthe front end of the recording medium at the timing when the recordingmedium is detected due to the fact that the recording medium collideswith the conveying rollers 307. That is, a configuration may be used inwhich, based on the position of the front end of the recording medium atthe timing when the recording medium is detected due to the fact thatthe recording medium collides with the conveying rollers 307, the timingwhen the front end of the recording medium reaches the nip position n iscalculated.

In the first, second, and third exemplary embodiments, the conveyingvelocity V is adjusted based on the distance X2 from the nip position nof the conveying rollers 307 to the detection position. Theconfiguration, however, is not limited to this. For example, theconveying velocity V may be adjusted based on the distance from the nipposition n of the conveying rollers 307 to a nip position of theregistration rollers 308. That is, the conveying velocity V may beadjusted based on the distance from the nip position n to apredetermined position downstream of the nip position n. Thepredetermined position is a position upstream of the transfer position.

In the first, second, and third exemplary embodiments, the number ofpairs of rollers from the conveying rollers 307 to the detectionposition is two. The configuration, however, is not limited to this. Forexample, three or more pairs of conveying rollers may be providedbetween the conveying rollers 307 and the detection position.

In the first, second, and third exemplary embodiments, the pickup roller303 or 305 is repeatedly rotated and stopped at predetermined timeintervals. The configuration, however, is not limited to this. Forexample, a configuration may be employed in which a swinging arm as aswinging member linking the pickup roller 305 and one of the feedingrollers 332 is supported by the rotating shaft of the feeding roller 332so that the swinging arm can pivot about the rotating shaft of thefeeding roller 332. Then, in the state where the rotational driving ofthe pickup roller 305 is continued, the pickup roller 305 is moved upand down at predetermined time intervals using the swinging arm, therebyfeeding recording media at predetermined intervals. In such aconfiguration, the CPU 151 a adjusts the conveying velocity V based ontime Tb from when the CPU 151 a outputs an instruction to move down thepickup roller 305 to when the sheet detector 700 outputs the signal ‘1’.

In the first, second, and third exemplary embodiments, a description hasbeen given of the method for adjusting the conveying velocity V of therecording medium fed by the pickup roller 305. The conveying velocity Vof the recording medium fed by the pickup roller 303 or 328 is alsoadjusted by a similar method.

In the first, second, and third exemplary embodiments, the conveyingvelocity V is adjusted based on whether the front end of the recordingmedium reaches the nip position n of the conveying rollers 307. Theconfiguration, however, is not limited to this. The conveying velocity Vmay be adjusted based on rollers other than the conveying rollers 307.For example, the conveying velocity V may be adjusted based on whetherthe front end of the recording medium reaches a nip position of theconveying rollers 322.

In the first, second, and third exemplary embodiments, the time Tc orthe distance Lc or Lc′ is set according to the grammage of the recordingmedium. The configuration, however, is not limited to this. For example,a configuration may be employed in which the time Tc or the distance Lcis set according to the rigidity or the thickness of the recordingmedium.

In the first, second, and third exemplary embodiments, the time Tc andthe distance Lc are set based on information regarding the paper typeinput by the user. The configuration, however, is not limited to this.For example, a configuration may be employed in which the time Tc andthe distance Lc are set based on the detection result of a sensor fordetecting the type of the recording medium, such as a thickness sensor.

In the first, second, and third exemplary embodiments, the threshold forthe deviation Δθ is a predetermined value, regardless of the paper type.Alternatively, the threshold may be set with respect to each paper type.

In the first, second, and third exemplary embodiments, if the absolutevalue of the deviation Δθ is greater than the threshold, the sheetdetector 700 outputs the signal ‘1’. If the absolute value of thedeviation Δθ is less than the threshold, the sheet detector 700 outputsthe signal ‘0’. The configuration, however, is not limited to this. Forexample, a configuration may be employed in which, if the absolute valueof the deviation Δθ changes from a value smaller than the threshold to avalue greater than or equal to the threshold, the sheet detector 700outputs the signal ‘1’ to the CPU 151 a.

A configuration may be employed in which the CPU 151 a has the functionof the sheet detector 700 according to the first, second, and thirdexemplary embodiments.

In the first, second, and third exemplary embodiments, the recordingmedium is detected by comparing the absolute value of the deviation Δθwith the threshold Δθth. The configuration, however, is not limited tothis. For example, the recording medium may be detected by comparing thecurrent value iq output from the coordinate transformer 511 with athreshold iqth. An increase in the current value iq means that the loadtorque applied to the rotor 402 of the motor 509 increases. A decreasein the current value iq means that the load torque applied to the rotor402 of the motor 509 decreases.

Alternatively, the recording medium may be detected by comparing, with athreshold iq_refth, the q-axis current instruction value (target value)iq_ref determined based on the deviation Δθ between the instructionphase θ_ref and the rotational phase θ determined by the phasedeterminer 513. An increase in the q-axis current instruction valueiq_ref means that a torque required for the rotation of the rotor 402 ofthe motor 509 increases due to an increase in the load torque applied tothe rotor 402. A decrease in the q-axis current instruction value iq_refmeans that the torque required for the rotation of the rotor 402 of themotor 509 decreases due to a decrease in the load torque applied to therotor 402.

Alternatively, a configuration may be employed in which the recordingmedium is detected by comparing the amplitude (magnitude) of the currentvalue iα or iβ in the stationary coordinate system with a threshold. Anincrease in the amplitude (magnitude) of the current value iα or iβ inthe stationary coordinate system means that the load torque applied tothe rotor 402 of the motor 509 increases. A decrease in the amplitudemeans that the load torque applied to the rotor 402 of the motor 509decreases.

The first, second, and third exemplary embodiments are applied not onlyto motor control by vector control. For example, the first and secondexemplary embodiments can be applied to any motor control device havinga configuration for feeding back a rotational phase or a rotationalvelocity.

In the first, second, and third exemplary embodiments, a stepper motoris used. as the motor for driving a load. Alternatively, another motorsuch as a direct current (DC) motor or a brushless DC motor may be used.The motor is not limited to a two-phase motor, and another motor such asa three-phase motor may be used.

In the vector control according to the first, second, and thirdexemplary embodiments, the motor 509 is controlled by performing phasefeedback control. The configuration, however, is not limited to this.For example, a configuration may be employed in which the motor 509 iscontrolled by feeding back a rotational velocity ω of the rotor 402.Specifically, as illustrated in FIG. 14, a velocity determiner 514 isprovided in the motor control device 157, and the velocity determiner514 determines the rotational velocity ω based on a change over time inthe rotational phase θ output from the phase determiner 513. Therotational velocity ω is determined using the following formula (16).

ω=dθ/dt  (16)

Then, the CPU 151 a outputs an instruction velocity ω_ref that indicatesa target velocity of the rotor 402. Further, a configuration is employedin which a velocity controller 600 is provided in the motor controldevice 157. The velocity controller 600 generates the q-axis currentinstruction value iq_ref and the d-axis current instruction value id_refso that the deviation between the rotational velocity ω and theinstruction velocity ω_ref becomes small. Then, the velocity controller600 outputs the q-axis current instruction value iq_ref and the d-axiscurrent instruction value id_ref. A configuration may be employed inwhich the motor 509 is controlled by performing such velocity feedbackcontrol. In such a configuration, a sheet is detected by the methodsdescribed in the first to third exemplary embodiments, for example,based on a deviation Δω between the rotational velocity ω and theinstruction velocity ω_ref. The instruction velocity ω_ref is a targetvelocity of the rotor 402 of the motor 509 corresponding to a targetvelocity of the peripheral velocity of the conveying rollers 307.

The deviations Δθ and Δω, the current value iq, the current valueiq_ref, and the amplitude of the current value iα or iβ in thestationary coordinate system correspond to the values of parameterscorresponding to the load torque applied to the rotor 402 of the motor509.

In the first and second exemplary embodiments, a permanent magnet isused as the rotor. The configuration, however, is not limited to this.

The configuration for detecting a sheet such as a recording medium isalso applied to, for example, a motor for rotationally driving aconveying belt. That is, the configuration for detecting a sheet isapplied to a motor for rotationally driving a rotating member, such as aroller or a conveying belt.

The photosensitive drum 309, the charging device 310, the developingdevice 314, and the transfer charging device 315 are included in animage forming unit.

In the first, second, and third exemplary embodiments, the registrationrollers 308 are used as an abutment member that the front end of therecording medium abuts so that the skew of the recording medium iscorrected. The configuration, however, is not limited to this. Forexample, a configuration may be employed in which a shutter as anabutment member is provided upstream of the registration rollers 308 anddownstream of the pre-registration rollers 333, or upstream of thetransfer position and downstream of the registration rollers 308 in theconveying direction of the recording medium. The front end of therecording medium is caused to abut the shutter, thereby correcting theskew of the recording medium by the above method. Then, when theregistration rollers 308 convey the recording medium to the transferposition in timing with a toner image, the shutter is retracted.

According to the exemplary embodiments of the present disclosure, it ispossible to prevent the situation where an image is formed at aninappropriate position on a sheet.

Embodiment(s) of the present disclosure can also be realized by acomputer of a system or apparatus that reads out and executes computerexecutable instructions (e.g., one or more programs) recorded on astorage medium (which may also be referred to more fully as a‘non-transitory computer-readable storage medium’) to perform thefunctions of one or more of the above-described embodiment(s) and/orthat includes one or more circuits (e.g., application specificintegrated circuit (ASIC)) for performing the functions of one or moreof the above-described embodiment(s), and by a method performed by thecomputer of the system or apparatus by, for example, reading out andexecuting the computer executable instructions from the storage mediumto perform the functions of one or more of the above-describedembodiment(s) and/or controlling the one or more circuits to perform thefunctions of one or more of the above-described embodiment(s). Thecomputer may include one or more processors (e.g., central processingunit (CPU), micro processing unit (MPU)) and may include a network ofseparate computers or separate processors to read out and execute thecomputer executable instructions. The computer executable instructionsmay be provided to the computer, for example, from a network or thestorage medium. The storage medium a include, for example, one or moreof a hard disk, a random access memory (RAM), a read-only memory (ROM),a storage of distributed computing systems, an optical disk (such as acompact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD™),a flash memory device, a memory card, and the like.

While the present disclosure has been described with reference toexemplary embodiments, it is to be understood that the disclosure is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2019-046342, filed Mar. 13, 2019, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An image forming apparatus comprising: a stackingunit on which a sheet is to be stacked; a pickup roller configured tofeed the sheet stacked on the stacking unit; a first conveying rollerconfigured to convey the sheet fed by the pickup roller; a transfer unitconfigured to transfer an image onto the sheet at an image formingposition downstream of the first conveying roller in a conveyingdirection in which the sheet is conveyed; a motor configured to drivethe first conveying roller; a determiner configured to determine a valueof a parameter corresponding to a load torque applied to a rotor of themotor; and a velocity adjuster configured to, based on a length betweena position of a front end of the sheet and a nip position of the firstconveying roller at a first timing when the value of the parameterdetermined by the determiner changes from a value smaller than apredetermined value to a value greater than the predetermined value, anda length between the nip position of the first conveying roller and apredetermined position downstream of the first conveying roller andupstream of the image forming position in the conveying direction,adjust a conveying velocity at which the sheet being conveyed at apredetermined velocity by the first conveying roller is conveyed to thepredetermined position.
 2. The image forming apparatus according toclaim 1, further comprising an acquisition unit configured to acquireinformation regarding a type of the sheet to be conveyed, wherein thevelocity adjuster adjusts the conveying velocity based on theinformation acquired by the acquisition unit.
 3. The image formingapparatus according to claim 1, wherein, in a case where a grammage ofthe sheet to he conveyed is a first grammage, the velocity adjusteradjusts the conveying velocity using a first length as the lengthbetween the position of the front end of the sheet and the nip positionat the first timing, and wherein, in a case where the grammage of thesheet to be conveyed is a second grammage greater than the firstgrammage, the velocity adjuster adjusts the conveying velocity using asecond length greater than the first length.
 4. The image formingapparatus according to claim 1, further comprising: a second conveyingroller provided upstream of the image forming position and downstream ofthe first conveying roller in the conveying direction; and an abutmentmember that is provided upstream of the image forming position anddownstream of the second conveying roller in the conveying direction andthat the front end of the sheet conveyed by the second conveying rollerabuts, wherein a skew of the sheet is corrected by abutment of the frontend of the sheet with the abutment member, and wherein the predeterminedposition is a position between the first and second conveying rollers.5. The image forming apparatus according to claim 1, further comprisinga controller configured to start and stop rotational driving of thepickup roller at predetermined time intervals, wherein the velocityadjuster adjusts the conveying velocity based on (i) time from a secondtiming when the rotational driving of the pickup roller is started to athird timing when the sheet is to reach the predetermined position, (ii)time from the second timing to the first timing, and (iii) the lengthbetween the position of the front end of the sheet and the nip positionand the length between the nip position and the predetermined positionat the first timing.
 6. The image forming apparatus according to claim1, further comprising: a swinging member configured to move up and downthe pickup roller that is rotationally driven; and a controllerconfigured to control the up-and-down movement of the swinging member,wherein the velocity adjuster adjusts the conveying velocity based on(i) time from a second timing when the rotational driving of the pickuproller is started to a third timing when the sheet is to reach thepredetermined position, (ii) time from the second timing to the firsttiming, and (iii) the length between the position of the front end ofthe sheet and the nip position and the length between the nip positionand the predetermined position at the first timing.
 7. The image formingapparatus according to claim 1, further comprising a feeding rollerprovided upstream of the first conveying roller in the conveyingdirection and configured to convey the sheet fed by the pickup rollerdownstream, wherein a conveying path which is provided between thefeeding roller and the first conveying roller and in which the sheet isguided is curved.
 8. The image forming apparatus according to claim 7,wherein the feeding roller is adjacent to the first conveying roller,and wherein a peripheral velocity of the first conveying roller isfaster than a peripheral velocity of the feeding roller.
 9. The imageforming apparatus according to claim 1, wherein, in a case where a statewhere the value of the parameter determined by the determiner is smallerthan the predetermined value continues for a predetermined time, theconveyance of the sheet is stopped.
 10. The image forming apparatusaccording to claim 1, wherein the determiner is a first determiner, theimage forming apparatus further comprising: a second determinerconfigured to determine a rotational phase of the rotor of the motor;and a controller configured to control a driving current flowing througha winding of the motor so that a deviation between the rotational phasedetermined by the second determiner and an instruction phase indicatinga target phase of the rotor becomes small.
 11. The image formingapparatus according to claim 10, wherein the controller controls thedriving current based on a torque current component that is a currentcomponent represented in a rotating coordinate system based on therotational phase of the rotor determined by the second determiner and isalso a current component that causes the rotor to generate a torque. 12.The image forming apparatus according to claim 1, wherein the determineris a first determiner, the image forming apparatus further comprising: asecond determiner configured to determine a rotational velocity of therotor of the motor; and a controller configured to control a drivingcurrent flowing through a winding of the motor so that a deviationbetween the rotational velocity determined by the second determiner andan instruction velocity indicating a target velocity of the rotorbecomes small.
 13. The image forming apparatus according to claim 12,further comprising a third determiner configured to determine arotational phase of the rotor of the motor, wherein the controllercontrols the driving current based on a torque current component that isa current component represented in a rotating coordinate system based onthe rotational phase of the rotor determined by the third determiner andis also a current component that causes the rotor to generate a torque.14. The image forming apparatus according to claim 1, further comprisinga detector configured to detect driving current flowing through awinding of the motor, wherein the determiner determines the value of theparameter based on the driving current detected by the detector.
 15. Animage forming apparatus comprising: a stacking unit on which a sheet isto be stacked; a pickup roller configured to feed the sheet stacked onthe stacking unit; a first conveying roller configured to convey thesheet fed by the pickup roller; a transfer unit configured to transferan image onto the sheet at an image forming position downstream of thefirst conveying roller in a conveying direction in which the sheet isconveyed; a motor configured to rotationally drive the first conveyingroller; a determination unit configured to determine a value of aparameter corresponding to a load torque applied to a rotor of themotor; an acquisition unit configured to acquire information regarding agrammage of the sheet to be conveyed; and a velocity adjustment unitconfigured to, according to a change of the value of the parameterdetermined by the determination unit changes from a value smaller than apredetermined value to a value greater than the predetermined value,adjust a conveying velocity at which the sheet conveyed at apredetermined velocity by the first conveying roller is conveyed to apredetermined position downstream of the first conveying roller andupstream of the image forming position in the conveying direction,wherein, in a case where the acquisition unit acquires informationindicating that the grammage of the sheet to be conveyed is a firstgrammage, the velocity adjustment unit adjusts the conveying velocity toa first velocity, and wherein, in a case where the acquisition unitacquires information indicating that the grammage of the sheet to beconveyed is a second grammage greater than the first grammage, thevelocity adjustment unit adjusts the conveying velocity to a secondvelocity faster than the first velocity.
 16. The image forming apparatusaccording to claim 15, further comprising: a second conveying rollerprovided upstream of the image forming position and downstream of thefirst conveying roller in the conveying direction; and an abutmentmember that is provided upstream of the image forming position and.downstream of the second conveying roller in the conveying direction andthat the front end of the sheet conveyed by the second conveying rollerabuts, wherein a skew of the sheet is corrected by abutment of the frontend of the sheet with the abutment member, and wherein the predeterminedposition is a position between the first and second conveying rollers.17. The image forming apparatus according to claim 15, furthercomprising a control unit configured to start and stop rotationaldriving of the pickup roller at predetermined time intervals, whereinthe velocity adjustment adjusts the conveying velocity based on (i) timefrom a third timing when the rotational driving of the pickup roller isstarted to a second timing when the sheet is to reach the predeterminedposition, (ii) time from the third timing to the first timing, and (iii)the length between the position of the front end of the sheet and thenip position and the length between the nip position and thepredetermined position at the first timing.
 18. The image formingapparatus according to claim 15, further comprising: a swinging memberconfigured to move up and down the pickup roller that is rotationallydriven; and a control unit configured to control the up-and-downmovement of the swinging member, wherein the velocity adjustment unitadjusts the conveying velocity based on (i) time from a third timingwhen the rotational driving of the pickup roller is started to a secondtiming when the sheet is to reach the predetermined position, (ii) timefrom the third timing to the first timing, and (iii) the length betweenthe position of the front end of the sheet and the nip position and thelength between the nip position and the predetermined position at thefirst timing.
 19. The image forming apparatus according to claim 15,further comprising a feeding roller provided upstream of the firstconveying roller in the conveying direction and configured to convey thesheet fed by the pickup roller downstream, wherein a conveying pathwhich is provided between the feeding roller and the first conveyingroller and in which the sheet is guided is curved.
 20. The image formingapparatus according to claim 19, wherein the feeding roller is adjacentto the first conveying roller, and wherein a peripheral velocity of thefirst conveying roller is faster than a peripheral velocity of thefeeding roller.
 21. The image forming apparatus according to claim 15,wherein, in a case where a state where the value of the parameterdetermined by the determination unit is smaller than the predeterminedvalue continues for a predetermined time, the conveyance of the sheet isstopped.
 22. The image forming apparatus according to claim 15, whereinthe determination unit is a first determination unit, the image formingapparatus further comprising: a second determination unit configured todetermine a rotational phase of the rotor of the motor; and a controlunit configured to control a driving current flowing through a windingof the motor so that a deviation between the rotational phase determinedby the second determination unit and an instruction phase indicating atarget phase of the rotor becomes small.
 23. The image forming apparatusaccording to claim 22, wherein the control unit controls the drivingcurrent based on a torque current component that is a current componentrepresented in a rotating coordinate system based on the rotationalphase of the rotor determined by the second determination unit and isalso a current component that causes the rotor to generate a torque. 24.The image forming apparatus according to claim 15, wherein thedetermination unit is a first determination unit, the image formingapparatus further comprising: a second determination unit configured todetermine a rotational velocity of the rotor of the motor; and a controlunit configured to control a driving current flowing through a windingof the motor so that a deviation between the rotational velocitydetermined by the second determination unit and an instruction velocityindicating a target velocity of the rotor becomes small.
 25. The imageforming apparatus according to claim 24, further comprising a thirddetermination unit configured to determine a rotational phase of therotor of the motor, wherein the control unit controls the drivingcurrent based on a torque current component that is a current componentrepresented in a rotating coordinate system based on the rotationalphase of the rotor determined by the third determination unit and isalso a current component that causes the rotor to generate a torque. 26.The image forming apparatus according to claim 15, further comprising adetector configured to detect driving current flowing through a windingof the motor, wherein the determination unit determines the value of theparameter based on the driving current detected by the detector.